This invention relates to ferroresonant magnetic circuits and, more particularly, to ferroresonant magnetic power supplies and techniques.
The power supply art has evolved over the years, and improvements in switching transistor technology and in transformer materials have had a particularly favorable effect in improving the performance and reliability of available power supplies. However, it is widely recognized that there is still much room for improvement in the efficiency, reliability and electromagnetic interference characteristics of modern power supplies.
A promising development in power supply technology is the use of zero current switching techniques, and the adaptation of such techniques for use in power supplies at high frequencies. Power supplies utilizing zero current switching have exhibited improved efficiency, reliability and EMI characteristics, but the complexity of many designs has been problematic. For example, in a zero current switching power supply, it is common to utilize frequency modulation for controlling the output. This adds complexity to the design of the power supply, and also has the disadvantage of unpredictability of the fundamental frequency, which can be a problem for certain applications, e.g. communications systems.
Power supplies employing a ferroresonant transformer are well known in the art. In many cases, the ferroresonant transformer is utilized for regulation purposes. Examples of such circuits, as well as other approaches using ferroresonant circuits can be found among the following U.S. Pat. Nos. 3,072,837, 3,098,193, 3,098,196, 3,101,450, 3,107,325, 3,117,274, 3,129,375, 3,241,035, 3,246,225, 3,323,039, 3,341,766, 3,349,317, 3,356,927, 3,374,423, 3,374,427, 3,404,329, 3,458,796, 3,521,147 and, 3,654,546.
Ferroresonant power supplies tend to be simple and reliable, and can operate from common supply frequency. They have extremely good input/output isolation and provide good regulation. Also, they can operate over a relatively wide range of input voltages. On the other hand, existing ferroresonant power supplies are generally large and heavy due to operation at relatively low frequency. Regulation, while often adequate, is not easily controllable. Also, output rail voltages tend to be dependent on supply frequency stability.
The ferroresonance principle involves the modulation of winding inductances on a cycle-to-cycle basis, and the transformer core operates in saturation mode during portions of the cycle. This factor has generally limited the frequencies at which ferroresonance power supplies can operate. If frequency of operation is raised beyond a certain point, core losses become severe. Even expensive core materials, which may have relatively better operating characteristics and temperature stability at higher frequency, still do not permit operation at as high a frequency as may be desirable for some designs.
It is among the object of the present invention to provide a ferroresonant power supply and method which can utilize a wide range of core materials and attain higher frequency operation than is presently available with a particular core material. It is also among the objects of the invention to provide a power supply with well controlled output regulation and to provide single frequency of operation despite load variations. It is among the further objectives of the invention to utilize zero current switching in a new type of power supply configuration, so as to obtain the advantages of zero current switching, but without the complexity attendant typical zero current switching designs.